This invention relates, in general, to power transistors, and more particularly, to power transistors with turn-off circuitry.
In general, a power transistor is used to drive a capacitive, inductive, or resistive load. A power transistor is used to deliver a current of a magnitude that cannot be provided by other circuitry. For example, an output stage on an integrated circuit may not have a drive capability required for an application. A buffer stage comprising at least one power transistor is coupled to the output stage to increase current drive. The output stage of the integrated circuit acts as a drive circuit for enabling and disabling the power transistor.
The speed at which a power transistor is turned on and off is related to an input capacitance of the power transistor and an output current of a drive circuit. A drive circuit enables and disables a power transistor and often must drive large capacitive loads. For example, an input capacitance of a power transistor such as a power MOSFET (Metal Oxide Semiconductor Field Effect Transistor) typically ranges from 1000-5000 picofarads. The speed at which a power transistor input capacitance is charged and discharged (for turning the power transistor on and off) is a function of a drive circuit output current. Many commonly sold integrated circuits do not have an output current capable of driving an input capacitance of a power transistor at high speeds which reduces power efficiency.
A power transistor is inefficient when operated with a drive circuit that cannot rapidly slew an input capacitance of the power transistor. The inefficiency occurs due to the increased time the power transistor remains on as it transitions from an on state to an off state. A power transistor dissipates power during a transition time period since it remains conducting until it is off. Power efficiency is increased by reducing the transition time period from the on state to the off state.
It would be of great benefit if a circuit could be integrated with a power transistor that substantially decreased the turn-off time of a power transistor thereby increasing power transistor efficiency.